The present invention is directed to the field of extrusion molding technology, particularly for fashioning large articles. Various molding processes are known for fashioning articles of polymer materials, such as blow molding. In a blow molding process, a “parison” (i.e. tube of extruded plastic) is extruded into a mold, and high-pressure air is blown into the mold to force the plastic to the edges of the mold cavity. It is known to use a continuous high-speed, high-volume blowing molding process for bottles. While such a blow molding process is perfect for making small parts like bottles, it is absolutely unsuited for making large plastic parts, such as a plastic pallet, because of its many limitations. When blowing a large plastic part, such as a plastic pallet, there will be too many areas of uneven stretching, especially for corners, edges and deep draws, resulting in extremely weak structural integrity. Also, in order to blow out a large part, the extruded parison in this process must be of a very large diameter, which creates certain physical limitations. The extruded parison will have walls too thick to be properly blown out by the blow molding process, and the concentrated weight of the plastic will create sagging and elongation. Also, a high level of inconsistency is encountered in the finished product.
It is often desirable to add color to a molded product. Under the standard process, color or pigment must be incorporated into the entire polymer. However, this approach results in excess cost since a pigment medium requires a higher quality plastic material, either virgin or high quality recycled plastic. Thus, the entire part must be made up of higher cost material, and it would not be possible to use a less expensive type of recycled plastic. It is known to use multiple layers in blow molding small parts such as bottles. For example, plastic bottles for food products may have an ultraviolet-blocking layer to preserve food freshness and increase shelf life of a product. It may also be desirable to add a colored pigment layer to a product to avoid the above-indicated difficulties. However, it is not feasible to use a multiple layer process in the manufacture of most large objects. It is known that such a multi-layer process can be successfully in making large objects that are of a simple, generally round shape, for example applying a color layer in the manufacture of trash cans and barrels. However, this process for making these objects is in most cases intermittent and not continuous, requiring considerable handling and special steps by manufacturing personnel.
It is known to manufacture large objects such as plastic pallets on a limited basis with a blow molding process. However, this process is also intermittent and non-continuous, requiring considerable handling and special steps by manufacturing personnel. Also, many other problems are associated with this process. The pallet-making process is necessarily a single-layer process, where the article can only be formed of a single layer of plastic, and does not allow using multiple layers. Thus it is difficult to obtain the benefits and flexibility that would result from controlling various parameters, including strength, weight, color, cost and level of anti-skid effectiveness, fire retardance, barrier protection, UV protection and recycle content of materials used. Also, since each part must be handled separately, and production is intermittent and not continuous, the plastic in the extruder and the extrusion header accumulates and overheats for an extended time while waiting for the next cycle, thus resulting in degradation of the plastic resin. Such pallets also suffer from the aforementioned problems with over-stretching at corners, edges and deep draws, and the extruded parison over-stretches under its own weight as it is coming down into the mold. Further, since the extrusion cycle is slow, the cooling of the extruded parison is uneven, with the lower end being progressively cooled at a much more excessive rate. This results in uneven blowing and an inferior surface finish.
Pallets manufactured in the conventional manner tend to have a smooth surface, and so extra steps must be taken to achieve anti-skid utility. It is common to spray or paint an anti-skid coating material such as epoxy, solvent or glue. Additionally, foreign materials like sand or rubber shreds are sometimes applied with the coating for better traction. It is also known to insert rubber grommets or strips into the product surface to provide an anti-skid surface. Also, it is known to machine the product surface to roughen it up and provide traction. These methods are performed after the product is made, resulting in extra manufacturing steps. They also result in other problems. In the coating method, epoxy, rubber, sand etc. are incompatible material with the plastic resin, and are not recyclable, as would be desired. The same is true with the rubber grommet and strip inserts, which are also very expensive and fall off easily. The surface machining method is again labor intensive and is not very effective as an anti-skid surface. Furthermore, a machine-roughed surface traps dirt and contaminants and is not believed to provide sufficient cleanliness to obtain USDA approval for food applications.
Similar difficulties are encountered in providing a fire-retardant plastic pallet. Fire-retardant material is very expensive, and the blended resin costs over four times that of a standard polymer without the fire-retardant additive. The resin with the fire-retardant additive is extremely heavy, making it undesirable and unsuitable for shipping and handling because of potential for injuries. Also, the resulting product with the additive is very brittle and breaks and cracks very easily, resulting in reduced strength and a short useful life. Because of the high quantity of additive required, the plastic part is not suitable for recycling, further limiting the incentive to a widespread usage.
Other issues are important in the manufacture of pallets and other large molded items. On one hand, it is important to provide as much mechanical strength as possible. On the other hand, it is desirable to provide impact resistance. This is often accomplished by making plastic products of structural foam. However, in the structural foam process the foaming has to be done throughout the product and may require a certain amount of sacrifice in rigidity and mechanical strength. In other applications, a barrier between the inside content and the outside environment is often a crucial feature in certain type of product. One obvious example is the gasoline tank used in an automobile. Barrier materials in almost all cases are very expensive. Moreover, depending on the type of barrier materials used, they could have problems with being too heavy or too rigid. Products for some markets may require an antistatic feature. Some of the common industries require an anti-static feature, such as the chemical industry where static can spark and thereby create a fire hazard, and the electronic industry with sensitive electronic components that static electricity may interfere or damage. Anti-static additives are again expensive and result in penalties in physical properties.
In a typical extruded polymer molding process (including blow molding, and other type of profile extrusion with hollow core,) it has been difficult to perform steps of in-filling of material(s) and/or fibers to the inside of column for enhanced physical properties, including higher strength, better impact resistance, better flexural properties, and higher tensile strength. The difficulties with such in-filling of material(s) and/or fibers limits the type of polymer products that can be manufactured and restricts the innovation and design of polymer products that could have been made as preferable alternative to the replacing of existing utilities, including the making of a polymer structural post or other types of polymer support structures.
The in-fill of foamed material offers polymer parts that are more impact resistant, high in insulation value, and high in tensile and flexural strength. However, in typical molding processes, the in-filling of materials including foamed material into the internal cavities of a molded product is a very cumbersome, costly, and slow process, and offers less than ideal quality in the finished product. A common process where in-filling of material generally takes place is rotational molding and/or urethane batch mixing and filling, which is an expensive and slow process.
In utilizing typical molding processes, it is known that when fibers (natural or synthetic) like fiberglass or hemp are introduced into polymer and sent through the molding machine system (such as but not limited to extruders, accumulator, injection port etc.,) the fibers have a natural tendency to align themselves in the same direction. This tendency for fibers to line up creates undesirable consequences, including the lowering of maximum strength, tensile and flex potential, and the increase in surface cracks and brittleness for the fibers embedded polymer.
In a typical molding process, it would be desirable if a color or clear stripe could be applied or embedded into a molded polymer product for identification or aesthetic purposes. However, such a striping system is not available for products that are manufactured under the typical extrusion molding processes as described above.